Transportation systems, particularly public transportation systems such as train, tram, underground, bus or coach systems rely on passengers paying a fare in order to travel. There are many different charging schemes that may be implemented, such as paying to travel a particular distance, a particular number of stops or to a particular location. The result is that in return for paying a particular fare, the passenger is entitled to travel on the vehicle to their desired destination.
Many transportation systems rely on conductors to ensure passengers have paid the correct fare for their journey. The conductor will patrol the vehicle checking to ensure each passenger's ticket is valid for the terms of their travel. The conductor may also have a ticket machine for issuing tickets. Tickets may be physically issued, in the form of card or paper tickets, or they may be provided electronically such as to a mobile phone, personal data assistant (PDA) or any other mobile device. Unfortunately it is difficult for a conductor to keep track of all the passengers, especially when the vehicle stops at a number of different locations along a route at which passengers can exit or enter the vehicle. This can result in passengers evading their fare by not buying a ticket, or travelling a further distance than their ticket entitles them to. Attempts to solve this problem include the use of barriers at train stations to only let through those with correct tickets, but the barriers still need to be supervised and there are still many train stations without ticket barriers.